1. Technical Field
This disclosure relates to charge accumulators such as batteries and capacitors, in particular to high capacity multiple cell batteries. More particular this disclosure relates to multiple cell batteries with improved stability to increase capacity of individual cells.
2. Background
U.S. Patent Application Publication 2010/0175245 Do; et al. illustrates an exemplary stacking structure a lithium ion battery. FIGS. 1a-1d illustrates the structure of an anode of a lithium ion battery cell of Do; et al. FIGS. 2a-2d illustrates the structure of a cathode of a lithium ion battery cell of Do; et al. FIG. 3 illustrates the organization of multiple current collectors that are commonly referred to as anodes and cathodes to form a lithium ion battery of Do; et al. In FIGS. 1a-1d, an anode 100 is formed of a metal film 120 such as copper and is coated with an active anode material 110 on both sides of the metal film 120 cut to predetermined dimensions. The active anode material 110 material is typically graphite or carbon that is coated in the form of slurry, and dried. The active anode material 110 does not coat the entire surface area of the metal film 120. An adhesive 130 is placed on an edge portion that is not coated with the active anode material 110 on one side of the anode 100. The adhesive 130 is a material such as glue, starch, an adhesive tape, or other adhesive means. The side of the anode 100 having adhesive material 130 is adhered to a separator 300. The separator 300 is generally a macroporous film of polyethylene or polypropylene.
In FIGS. 2a-2d, a cathode 200 is formed of a metal film 220 such as aluminum and is coated with an active cathode material 210 on both sides of the metal film 220 cut to predetermined dimensions. The active cathode material 210 is typically Lithium cobalt oxide (LiCoO2). Lithium Iron Phosphate (LiFePO4), or other similar reactive material. The active cathode material 210 does not coat the entire surface area of the metal film 220. An adhesive 230 is placed on an edge portion that is not coated with the active cathode material 210 on one side of the anode 200. The adhesive 230 is a material such as glue or starch, an adhesive tape, or other adhesive means. The side of the cathode 200 having adhesive material 230 is adhered to a separator 300.
In FIG. 3, the anodes 100 attached to a separator 300, the cathodes 200 attached to a separator 300, an unattached anode 100 have their an adhesive placed at the remaining edge portion not coated with the active anode material 110 or active cathode material 210. The unattached anode 100 is placed at a bottom of the stack and the cathodes 200 attached to the separators 300 are stacked alternately with the anodes 100 attached to the separators 300 in an interleaved fashion. The stack is then compressed to adhere the adhesives to an opposite side of the separator 300 placed above the electrode (anode 100 or cathode 200). The compressed stack 400 is then placed in a shell (not shown) and an electrolyte solution is placed in the shell to immerse the anodes 100 and the cathodes 200. The electrolyte solution is a mixture of a solute (as ion source) dissolved in organic solvent. In lithium ion batteries, propylene carbonate or ethylene carbonate may be used as the organic solvent and lithium phosphate as the solute.
In some implementations, each electrode (anode 100 and cathodes 200) is a sheet of electrically conductive current collector, such as copper film in the anodes 100, and aluminum film in the cathodes 200. Except for the top and bottom sheets in the stack, each electrode 100, 200 is coated on both sides with electrochemically active material, such as graphite in the anodes 100 and lithium metal oxide in the cathodes 200. The top and bottom surfaces of the stack do not require coating, because these two surfaces are not facing electrodes 100, 200 of opposite polarity and do not participate in the electrochemical reaction.
Internal shorts between the electrodes 100 and 200 of a cell of a battery may be caused by burrs on the current collector (the aluminum or copper foil on which the electrochemically active material is coated). In particular, aluminum burrs may penetrate the separator and contact the graphite to cause thermal runaway. The contact resistance closely matches the internal resistance (output resistance) of a lithium-ion cell for portable electronics.
Another cause of the internal shorts is mossy lithium dendritic growth on the anode surface may penetrate the separator. Since the dendrite will contact lithium metal oxide cathode coating, which is not very conductive, the hazard of thermal runaway is lower than aluminum burrs. It takes a very large mossy lithium dendrite to initiate thermal runaway.
Foreign particles such as conductive metal chips present in incoming material (for example steel wear debris in the cathode powders, or falling burrs from the steel cans), or entering the cell during assembly process (for example wear debris in the conveyer) may migrate into the cathode/anode interface. Also, metal particles formed during charging/discharging cycles by galvanic process and can thus breach the separator to cause shorts.
External forces such as bending and impact on the cell may cause electrodes to break. Exposed current collectors (Cu or Al foils) may penetrate the separator to cause shorts.
Manufacturing defect such as a defective separator or folding separator during the stacking/winding (assembly) process may leave the positive and negative electrodes in contact over the defect zone.
Usually, internal short involves only one interface. In a conventional cell, all positive/negative electrode interfaces are hard wired together. Thus the short draws current from all interfaces. In the present disclosure, the interfaces are partitioned into two groups which may be separated completely or through a buffer resistor. Thus only half of the interfaces are contributing to the short current. Considering that the heat generated at the short spot is proportional to the square of current, this disclosure effectively cuts down the hot spot heat generation by a factor of four.
When an inadvertent electrical short circuit occurs within a cell, the resistance of the short circuit connection causes overheating in the electrode and the separator. The temperature at the short circuit region may be sufficiently high to trigger thermal runaway which will lead to fire and explosion. For any given cell, the short circuit connection generates maximum amount of heat when its resistance equals the cell's output resistance. However at any given short circuit, heat generation increases monotonically with decreasing output resistance of the cell. Since the output resistance is approximately inversely proportional to the cell capacity, heat generation at any given short increases monotonically with the cell capacity.
The electrical short circuiting of the electrodes is often referred to as a “dead short”, i.e., bare metal contacts with negligible resistance. In this case, the short circuit current is proportional to the cell capacity. By Joule's law, heat generation at the short is proportional to the short circuit current squared, hence the square of cell, capacity. A dead short circuit in a high capacity cell or battery can generate enough heat to induce thermal runaway around the short circuit location within the electrodes, even if the short circuit is very brief (e.g., the contact may burn out or melt down). Thus the threshold of cell capacity must be observed very strictly.
Thermal runaway occurs when heat released in a short circuit region through exothermal chemical reaction exceeds heat dissipation capability from the short circuit region. Thus cooling is an effective way for the prevention of the exothermic reaction to cause the thermal runaway. Experience has shown that thermal runaway occurs much more readily at elevated temperature than at room temperature. Furthermore, cooling by conduction is known to reduce the occurrence of thermal runaway. For example, a short at the corner of an electrode is more likely to cause thermal runaway than a short at the center of the electrode. This is because a short circuit at a corner is connected to electrodes in only one quadrant, where as a short at the center is surrounded by electrodes in all four quadrants. A short at the center has four times as much heat sink as a short at a corner.
A larger cell does not necessarily serve as a better heat sink to a short, because the thermal runaway may begin rapidly in a small region, especially during a dead short. There is no significant heat dissipation outward. Thus it does not matter how large the cell is, as far as heat sink is concerned. Cell output resistance remains as the dominant factor in the initiation of thermal runaway. Unfortunately, the battery output resistance is getting lower and lower to meet the power requirement of modern portable electronics.
In order to prevent thermal runaway, a common practice is to partition the battery into a plurality of low capacity cell types. Presently two or three cells are connected in parallel in popular tablet computers such as Apple iPad™ Merging two cell types into one may result in 12% to 19% increase in capacity, and 15% cost reduction. However such cost reduction and capacity gain cannot be realized, lest the capacity of such merged cell may exceed the safety threshold. There is an acute need to raise the safety threshold.
Modest cost reduction can also be achieved by encasing multiple cells into a single shell. Amperex Technology Limited (ATL), Tsuen Wan, N. T., Hong Kong (Assignee of the present disclosure) has been shipping such shell-sharing cells since 2008. For example, each 20 Ah battery cell consists of a single stainless steel case, with two shell-less cells (commonly known as “jellyrolls”) stacked together, and connected internally in parallel. U.S. patent application Ser. No. 12/694,144 (Ramesh et al.) provides a battery pouch holding multiple jellyrolls side-by-side. The multiple jellyrolls are connected outside of the pouch. Heat transfer between multiple jellyrolls is ineffective in ATL's stacking arrangement, and negligible in Ramesh's side-by-side layout.